FIG. 1 shows a structure of protection of an electronic circuit connected to a telephone line corresponding to FIG. 2 of U.S. Pat. No. 8,687,329 (incorporated by reference). An electronic telephone signal transmit and receive circuit 1, or SLIC (“Subscriber Line Interface Circuit”) is connected to a telephone line formed of two conductors 3 and 5, at voltages VTIP and VRING. Abrupt overvoltages, for example due to lightning, may occur on conductors 3 and 5 and may damage circuit 1. Conductors 3, 5 are connected to a protection structure 7 capable, when the voltage on one of the conductors comes out of an interval defined by two threshold voltages, of discharging the overvoltage towards a ground 9. The voltage thresholds are defined by power supply voltage sources 11 of positive potential VH and 13 of negative potential VL. Protection structure 7 comprises two cathode-gate thyristors 15 and 17 having their cathodes respectively connected to conductors 3 and 5, and having grounded anodes. The gates of thyristors 15 and 17 are respectively connected to the emitters of two transistors 19 and 21 of NPN type, having their collectors connected to ground 9 and their bases connected to power supply source 13 of negative potential VL. Protection structure 7 also comprises two anode-gate thyristors 23 and 25 having their anodes respectively connected to conductors 3 and 5 and having their cathodes connected to ground 9. The gates of thyristors 23 and 25 are respectively connected to the emitters of two PNP-type transistors 27 and 29, having their collectors connected to ground 9 and their bases connected to power supply source 11 of positive potential VH.
In normal operation, the voltages of conductors 3 and 5 remain between VL and VH. All transistors are off, as well as all thyristors.
In case of a negative overvoltage on conductor 3, lower than negative potential VL, the potential of the base of transistor 19 becomes greater than the potential of its emitter, and transistor 19 turns on, which turns on thyristor 15. All along the overvoltage on the line, thyristor 15 remains on and discharges the overvoltage towards ground 9.
In the case of a negative overvoltage lower than negative potential VL on line 5, the operation is the same as that described for the case of a negative overvoltage on line 3, and implies thyristor 17 and transistor 21.
Similarly, in the case of a positive overvoltage higher than positive potential VH appearing on line 3 or 5, the operation is similar to the case of a negative overvoltage. A positive overvoltage on line 3 implies anode-gate thyristor 23 and PNP transistor 27. A positive overvoltage on line 5 implies anode-gate thyristor 25 and PNP transistor 29.
After the end of an overvoltage, the implied thyristor only turns off when the current flowing therethrough becomes lower than its hold current. The hold current of the thyristors should thus be higher than the maximum current capable of flowing through the telephone line. The maximum current is, for example, in the order of 150 mA. To obtain high hold currents, the thyristors are provided with emitter short-circuits, such as for example described in U.S. Pat. No. 5,274,524 (incorporated by reference).
A disadvantage of emitter short-circuit thyristors is that they have a low sensitivity, that is, they require a high gate current to turn on. Further in the absence of overvoltage, no current should be able to flow between the protection structure and the conductors of the telephone line, which have voltages in the range from VL to VH. Now, in each of the thyristors, the presence of emitter short-circuits enables a current to flow between the gate and the conductor of the telephone line connected to the thyristor.
Accordingly, a transistor is provided so that the junction between the emitter and the base of the transistor blocks the flowing of a current in the absence of an overvoltage. This transistor is also used to amplify the current supplied by the power supply sources of potentials VL and VH to reach the gate current necessary to turn on the thyristors.